CVE-2025-68291

In the Linux kernel, the following vulnerability has been resolved:

mptcp: Initialise rcvmss before calling tcpsendactivereset() in mptcpdofastclose().

syzbot reported divide-by-zero in _tcpselect_window() by MPTCP socket. [0]

We had a similar issue for the bare TCP and fixed in commit 499350a5a6e7 ("tcp: initialize rcvmss to TCPMIN_MSS instead of 0").

Let's apply the same fix to mptcpdofastclose().

CPU: 0 UID: 0 PID: 6068 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025 RIP: 0010:_tcpselectwindow+0x824/0x1320 net/ipv4/tcpoutput.c:3336 Code: ff ff ff 44 89 f1 d3 e0 89 c1 f7 d1 41 01 cc 41 21 c4 e9 a9 00 00 00 e8 ca 49 01 f8 e9 9c 00 00 00 e8 c0 49 01 f8 44 89 e0 99 <f7> 7c 24 1c 41 29 d4 48 bb 00 00 00 00 00 fc ff df e9 80 00 00 00 RSP: 0018:ffffc90003017640 EFLAGS: 00010293 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88807b469e40 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 RBP: ffffc90003017730 R08: ffff888033268143 R09: 1ffff1100664d028 R10: dffffc0000000000 R11: ffffed100664d029 R12: 0000000000000000 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 FS: 000055557faa0500(0000) GS:ffff888126135000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f64a1912ff8 CR3: 0000000072122000 CR4: 00000000003526f0 Call Trace: <TASK> tcpselectwindow net/ipv4/tcpoutput.c:281 [inline] _tcptransmitskb+0xbc7/0x3aa0 net/ipv4/tcpoutput.c:1568 tcptransmitskb net/ipv4/tcpoutput.c:1649 [inline] tcpsendactivereset+0x2d1/0x5b0 net/ipv4/tcpoutput.c:3836 mptcpdofastclose+0x27e/0x380 net/mptcp/protocol.c:2793 mptcpdisconnect+0x238/0x710 net/mptcp/protocol.c:3253 mptcpsendmsgfastopen+0x2f8/0x580 net/mptcp/protocol.c:1776 mptcpsendmsg+0x1774/0x1980 net/mptcp/protocol.c:1855 socksendmsgnosec net/socket.c:727 [inline] _socksendmsg+0xe5/0x270 net/socket.c:742 _syssendto+0x3bd/0x520 net/socket.c:2244 _dosyssendto net/socket.c:2251 [inline] _sesyssendto net/socket.c:2247 [inline] _x64syssendto+0xde/0x100 net/socket.c:2247 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xfa/0xfa0 arch/x86/entry/syscall64.c:94 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x7f66e998f749 Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ffff9acedb8 EFLAGS: 00000246 ORIGRAX: 000000000000002c RAX: ffffffffffffffda RBX: 00007f66e9be5fa0 RCX: 00007f66e998f749 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003 RBP: 00007ffff9acee10 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001 R13: 00007f66e9be5fa0 R14: 00007f66e9be5fa0 R15: 0000000000000006 </TASK>